In my U.S. patent application, Ser. No. 839,076, entitled "Capacitor Charging Circuit", filed on Oct. 3, 1977, a circuit is described for more efficiently and more quickly charging a capacitor to a high voltage from a low voltage small capacity direct current source, such as a small battery. The circuit includes an inverter coupled to the battery for applying pulses of current to the primary winding of a transformer. The secondary winding is coupled to the rectifier that charges the capacitor. Means are provided for regulating the current in the primary winding to a constant value as long as the battery is capable of supplying it. Operation of the defibrillator requires the display of ECG waveforms on an oscilloscope that is powered from the battery. As long as the battery is fresh, it can supply sufficient voltage for the operation of the oscilloscope, but when too much of its energy has been discharged, its output voltage drops to a point where the oscilloscope becomes inoperative. This is due to the fact that the fixed value of regulated current provided by the circuit of the aforementioned patent application produces too much voltage drop through the increased internal resistance of the battery.